Correct vehicle suspension and geometry improve the directional stability, tracking, cornering, and the overall "driveability" and handling of a vehicle while also increasing tire tread life and wear. Wheel alignment is an important aspect of overall vehicle suspension and geometry. Wheel misalignment has typically been one of the leading defects of newly assembled vehicles and has been, as a result, one of the biggest after-sale warranty make up items. Vehicle manufacturers continue to seek an assembly line solution for auditing and adjusting vehicle wheel alignment before the vehicle is delivered to a customer. Toe angle and camber angle are measurements of two of the forces relating to proper vehicle wheel alignment.
Toe may be defined as the slanting of the wheels towards the front or back. A slanting of the wheels towards the front so that they are closer together at the front than the back is referred to as "toe in" and is indicated by a positive toe angle between (a) a line drawn in the plane of rotation of the wheel through the center of the wheel and parallel to the ground, and (b) a reference line drawn from the front to back of the vehicle and parallel to the ground. Where the wheels are farther apart at the front than at the back, the condition is "toe out" and is indicated by a negative toe angle.
Camber may be defined as the sloping of the wheels from top to bottom. A sloping of the wheels inwards towards the bottom so that they are closer together at the bottom than at the top results in an outward tilt of the tires at the top. This is indicated by a positive camber angle between (a) a vertical line drawn in the plane of rotation of the wheel through the center of wheel, and (b) a reference line drawn perpendicular to the ground. Where the wheels are farther apart at the bottom than at the top, the condition is indicated by a negative camber angle.
Correct vehicle wheel alignment results in a better handling vehicle with reduced tire wear. It does so by helping to ensure, by toe and camber adjustment, that the wheels are within the automobile manufacturer's specifications for geometrical position and that the roll of the tire is within the driver's expectations for driveability. There are two distinct, and commercially competing, kinds of devices currently being used by vehicle manufacturers and others to measure and adjust toe and camber. The two kinds of devices may be referred to as geometric aligners and dynamic aligners.
A geometric aligner measures toe or camber, using angles derived from measurements taken from the sidewalls of the tires. A typical geometric aligner uses a system of rollers which contact the sidewalls of the tires. Two rollers may be placed at either end of a rocker bar with the rocker bar pivotally pinned to a support rod so as to form a "T". The rollers are located at the ends of the horizontal bar across the top of the T, and the pivot point is located at the intersection of the horizontal and vertical member of the T. When the rocker arm is oriented horizontally with each roller in contact with the sidewall of the tire at opposite points on the circumference, an angle formed at the pivot point can measure toe in or toe out. As the rollers come in contact with the sidewall of the rotating tire, they adjust to reflect the position of the tire. The position of the rocker arm angle is then measured to give the toe angle of the wheel.
To determine the camber of the wheel with a geometric aligner, a third roller is generally placed perpendicularly to the other two. From the position of this third roller relative to the other two, the camber angle may be derived. Because of wheel run out, tire carcass deviations, and raised lettering, the tire's sidewall surface is not uniform. As a result, most geometric alignment systems will require several revolutions of the tire to obtain a continuous average of sampled data points from which to derive the toe and camber angles.
A dynamic aligner, in contrast, derives toe and camber angles, not from the rotating sidewall of the tire, but from the tread of the rotating tire using a servomechanism to turn and tip a displacement box beneath the tire. A dynamic aligner is designed to have a pair of rollers underneath the tires of a vehicle. The rollers are typically mounted in a displacement box which, by means of a servomechanism, can be made to pivot about a toe axis and also made to pivot about a camber axis. As the tire rotates over the surface of the pair of rollers, the roller pair will be displaced. When both rollers are displaced in the same direction along their axes, a toe adjustment is indicated, and the servomechanism will cause the displacement box to turn in toe until the rollers' axes are parallel to the tire's axis of rotation and perpendicular to the toe plane, creating an angle that measures toe in or toe out. When the rollers are displaced in opposite directions along their axes, a camber adjustment is indicated, and the servomechanism will cause the displacement box to tip in camber until the rollers' axes are parallel to the tire's axis of rotation and perpendicular to the camber plane, creating an angle that measures camber. At the point where there is no further displacement of the rollers, a null, or steady state is reached, and the toe and camber angles may be read. The camber servomechanism's null state is crucial to dynamic alignment for, without it, unbalanced camber forces would cause a misreading of the toe angle.
It is possible to construct a type of dynamic aligner which does not yield the "full" dynamic alignment function as described above, that is, the full dynamic alignment which includes the turning and tipping of the aligner in both the toe and camber planes. Specifically, another type of dynamic aligner could be constructed which turns only in the toe plane. In this other type of dynamic aligner, an approximation of the tire force condition could be achieved by gimbaling (and measuring) only the toe plane while fixing the camber plane to the nominal camber value of the vehicle to be tested. The approximation achieved could be read as a dynamic toe inclination and could be used to derive an approximation of the dynamic toe angle. It should be understood that, while the following discussion of the system and method of kinetic alignment describes a full dynamic aligner coponent, the system and method is readily adaptable to any other type of dynamic aligner, and that the workings of other types of dynamic aligners are included within the description of a full dynamic aligner.
Each of the aligner types, geometric and dynamic, is well known. There are at least four vendors currently offering geometric type aligners, including Hoffmann (part of the Wegmann Group, 3700 Cohen Pl., P.O. Box 10369, Lynchburg, Va.) and Hunter (Hunter Engineering Co., 11250 Hunter Drive, Bridgeton, Mo.). It is believed that MERILab, Inc., the assignee of the present invention, is the only vendor currently offering a dynamic aligner. Among the published descriptions of dynamic and geometric alignment devices are those contained in U.S. Pat. No. 3,187,440 of Merrill, et al., assigned to Merrill Engineering Laboratories, Inc., the predecessor of the assignee of the current invention (describing a dynamic aligner); U.S. Pat. No. 4,380,875 of Erickson, et al., assigned to the predecessor of the assignee of the current invention (containing additional description of a dynamic aligner); and U.S. Pat. No. 4,856,199 of Merrill, et al., assigned to the predecessor of the assignee of the current invention (describing a geometric aligner, modified to have a single contact point).
It is commonly believed that geometric and dynamic aligners each have certain distinct advantages and disadvantages.
A geometric aligner's perceived advantages follow from its ability to measure the plane of rotation of the wheel from the sidewall of the tire and, hence, to compare the toe and camber angles derived from the measured plane of rotation against the manufacturer's specification. Vehicle manufacturers typically write their specifications geometrically, specifying the toe and camber angles the plane of rotation of the wheel should describe relative to the respective reference lines. When a vehicle manufacturer desires to audit its own assembly line performance in comparison to its specifications for wheel position, the geometric aligner is considered to provide a useful, objective way of measuring quality control.
Two commonly perceived disadvantages of geometric aligners also follow from its sidewall orientation. In the first place, because the sidewall of a tire is uneven, accurate measurement of wheel position from the side of the tire typically requires at least one complete revolution of the tire during which revolution several data points are sampled; for more complete accuracy, the data points are typically averaged, sometimes using statistical algorithms to determine whether a sufficient number of data points have been obtained. During a wheel alignment operation, the geometric aligner is slower than the operator--remembering that the geometric alignment takes some discrete time for wheel revolution and some discrete time for data sampling and analysis, the actual sequential operation can bog down. The operator will typically take an initial measurement of wheel alignment, then make an incremental adjustment to wheel position, then take a second measurement, then make another incremental adjustment, and so on until the resulting measurement is within the proper range. After every incremental adjustment, the operator must wait for the geometric aligner to catch up.
Although there are indications that geometric aligners can, in fact, be expected to function in the near future much more rapidly than they have functioned in the past, it has typically been the case that geometric alignment has been perceived to to too slow for complete assembly line audit. Instead, it has not been unusual for an assembly line to sample only a few vehicles per shift for geometric alignment checking.
In the second place, some persons have contended that geometric alignment tends to be overly theoretical insofar as it is designed to measure tolerance to design specifications rather than to measure wheel alignment with respect to expected road handling considerations. Such persons tended to suggest that dynamic aligners were more likely to produce a properly handling vehicle.
The perceived advantages and disadvantages of dynamic aligners are, in a sense, the converses of the geometric aligners. A dynamic aligner's perceived advantage follows from its ability to measure displacement from the tread of a rotating tire. As the tire rotates over a pair of rollers, the rollers themselves are displaced on their axes within a displacement box. A servomechanism causes the displacement box to turn about a toe axis and to tip about a camber axis until the box comes into line with the tire forces and the displacement of the rollers reaches a null state for both toe and camber. The ability of the servomechanism to tip the displacement box until a null state for camber is reached preserves the integrity of the resulting toe measurement by eliminating what would otherwise be an unbalanced camber force vector affecting the toe forces. The displacement of the rollers is the starting point for the servomechanism's turning or tipping the displacement box which, in turn, permits the derivation of toe and camber angles. The action of the tire tread on the dynamic rollers approximates the action of the tire tread on the road. When a manufacturer desires to adjust toe and camber to produce an alignment that the customer will perceive to be correct, the dynamic aligner is considered to provide a useful way of increasing customer satisfaction.
Further, because the dynamic aligner works on displacement at the point of tread contact, it can produce accurate readings very rapidly. Typically, a dynamic aligner must initially be calibrated to compensate for tire "run out" during one or more complete revolutions of the tire. But, from that point on, a dynamic aligner can produce its readings from a small arc of rotation. As a result, during a wheel alignment operation, the dynamic aligner is perceived to respond to the operator's adjustments as rapidly as they are made. A typical dynamic aligner has been much more rapid than a typical geometric aligner, and real time dynamic alignment has been possible on an assembly line basis with every vehicle on the line being tested and aligned.
The commonly perceived disadvantage of dynamic aligners also follows from the dynamic aligner's tire force orientation. It is contended that dynamic wheel alignment measurement is not intended to be, and is not, an objective measure of the vehicle's wheel position relative to manufacturer's specifications. So, it is contended, the dynamic wheel alignment measurements do not permit a vehicle manufacturer to learn anything about the quality control of its vehicle assembly operations.
It is not the intent of this discussion of the background of the invention to do more than outline the contours of some of the major difficulties facing vehicle manufacturers and those who have attempted to produce effective wheel alignment devices. For now, the important point is simply the existence of two distinct kinds of measuring devices, each of which has been commonly perceived to have its own unique advantages. To be properly aligned, a vehicle's toe and camber must be set to be within the vehicle manufacturer's specifications, but the vehicle must also drive properly.
Because geometric alignment is taken from the sidewall of the tire and is designed to calculate the plane of rotation of the wheel itself, geometric alignment is generally believed to set the alignment more nearly in accordance with the vehicle manufacturer's specifications for wheel position. Because dynamic alignment is taken from the rolling tread of the tire and is designed to calculate the displacement of the tire as it tracks on the ground, dynamic alignment is believed to set the alignment more nearly in accordance with the vehicle driver's expectations for driveability.
If the two devices produced identical readings, and were otherwise equal, it would seem likely that one would have supplanted the other. This has not happened. The devices produce different readings (a vehicle wheel's toe angle measured on a geometric aligner can, and frequently does, differ from the same vehicle wheel's toe angle measured a dynamic aligner). It is believed that the two devices produce different readings because a dynamic aligner is sensitive to tire forces to which a geometric aligner is not responsive.
Tire forces, including conicity and ply steer, are evident where the tread of the tire hits the road and can apply lateral force to pull a wheel sideways. Properly constructed tires should have tire forces within tire specifications so that there is no, or minimal, tire force effect on the driveability of a vehicle. Accordingly, where the tires are perfectly constructed, there should be no difference between geometric and dynamic measurements of the wheel alignment of a vehicle, but where tires are not so constructed there will be a difference between the readings equal to the amount of unbalanced tire force.
It is possible, because of tire forces attributable to tires out of specification, for a vehicle wheel to be perfectly aligned in accordance with geometric toe and camber, and yet, at the same time, drive as if out of alignment. It is in such conditions that geometric and dynamic measurements of wheel alignment will differ, with one measuring the specifications of the wheel and the other measuring the driveability of the wheel.
It is a specific object of the present invention to create an alignment system which combines the best elements of geometric aligners and dynamic aligners to create results unobtainable from either device alone. Specific advantages of this invention are speed, flexibility, self-audit and tire audit, all being done at a production line through-put rate that is relatively high compared to present alignment systems and at no added labor cost. In particular, the kinetic system and method of this invention: obtain the level of speed previously associated with dynamic alignment; permit geometric and dynamic measurements to be taken nearly simultaneously and enable an operator to make alignment adjustments using either measure as base; record both geometric and dynamic data for self-audit purposes; and, perhaps most importantly, permit a tire audit to be done on the same production line cycle with no additional time being required to check the tires.
It is another object of the present invention to create an improved geometric aligner to be used as part of the kinetic alignment system of this invention, or otherwise. The improved geometric aligner of this invention has (a) an improved pivot assembly so as better to keep the rollers of a geometric aligner in contact with the tire sidewall as the tire's position changes, and (b) an improved striker assembly so as better to strike the sidewall of the tire at the tire's center line and, where the exact center line is missed, to self-correct for the unbalanced forces resulting from the missed strike. The improved geometric aligner contributes to the overall advantages of the kinetic alignment system of this invention.